1. Field of the Invention
The present disclosure relates to wireless communication and, more particularly, to hybrid automatic repeat request and channel information feedback for a relay in a wireless communication system.
2. Description of the Related Art
ITU-R (International Telecommunication Union-Radio communication sector) is proceeding with a standardization of an IMT (International Mobile Telecommunication)-Advanced, a next-generation mobile communication system following the 3rd-generation. The IMT-Advanced aims to support IP-based multimedia services at a data transfer rate of 1 Gbps in a pause and low-speed movement state and at a data transfer rate of 100 Mbps in a high-speed movement state.
3GPP (3rd Generation Partnership Project) is preparing an LTE-A (Long Term Evolution-Advanced) as a system standard satisfying the requirements of the IMT-Advanced. The LTE, part of E-UMTS (Evolved-UMTS) using E-UTRAN (Evolved-Universal Terrestrial Radio Access Network), by the 3GPP standardization organization, employs OFDMA (Orthogonal Frequency Division Multiple Access) in downlink and SC-FDMA (Single Carrier-Frequency Division Multiple Access) in uplink. The LTE-A has evolved from the LTE.
The related art wireless communication system such as LTE includes a mobile station (MS) and a base station (BS) providing a service to a cell, a certain area. The quality of a transmission signal is affected by a change in a radio environment in terms of the wireless transmission technique. In particular, a radio channel changes over time due to various types of scatters, the movement of the MS, and the like. In addition, as the distance between radio communication subjects increases, reception power is rapidly reduced, so the distance is restricted. Thus, in general, the MS can communicate with the BS when it is within the coverage of the BS. A maximum transmission speed between the BS and the MS, throughput of users within a cell, and throughput of the entire cell tend to be reduced due to the factors such as the scatters, the movement speed of the MS, the transmission/reception distance, and the like. For example, when the MS is located at a cell boundary, or when an obstacle such as a building or the like is present between the MS and the BS, communication quality between the MS and the BS may not be good.
In an effort to solve such problem, various techniques for compensating degradation of a transmission signal between the BS and the MS can be introduced to obtain effects such as a maximum transmission speed, the improvement of throughput, an extension of coverage, and the like. One of those techniques is introducing of a relay station (RS) in the wireless communication system. The RS is capable of extending the coverage of the BS and enhancing a cell throughput. The LTE-A may include such RS.
When the wireless communication system includes the RS, a link between the BS and the RS and a link between the RS-RS and MS should be all considered.
In addition, in the wireless communication system, the BS, the RS, and the MS may use multiple antennas. When the number of transmission antennas is nT and the number of reception antennas is nR in the link between BS and RS and the link between the RS and a relay UE (Re UE), a channel matrix H of each link has a size of nR×nT, and a maximum number ‘r’ of a transmission codeword (CW) (referred to as a ‘codeword’, hereinafter) that can be simultaneously transmitted at the same frequency theoretically when there is no correlation between antennas is determined by equation shown below:r=rank(H)=min(nT,nR)≦nR 
For example, if the BS uses four transmission antennas and the RS uses four reception antennas, the rank of the link between BS-RS is 4, and maximum four codewords can be simultaneously transmitted or received (here, the rank corresponds to the number of layers).
When the rank is different or the amount of traffic is not balanced in the link between the BS and the RS or in the link between the RS and the Re UE, a method for relaying data by the RS without wasting radio resource is required, and a channel information feedback method in association therewith is also required. In addition, in the above-described case, if the RS does not have a function of performing scheduling by itself, how it performs HARQ matters.